The problem of weight gain and obesity is important in young adulthood. As individuals traverse life paths from adolescence into early adulthood, whether or not they attend college, join the military or enter the workforce out of high school, they encounter multiple stressors and influences that contribute to weight gain. Weight gain in turn leads to increase risk of cardiovascular disease, diabetes and other health problems. Little is known about how to intervene to prevent weight gain or enable weight loss in this population. We propose to conduct formative research to develop an intervention, Social/Mobile Approach to Reduce Weight (SMART) to prevent weight gain and promote weight loss in young adults at risk for weight gain. We will then conduct a randomized controlled trial in 406 participants to evaluate the effects on weight status and other metabolic, behavioral and psychosocial outcomes of SMART at 12 and 24 months. SMART will be a theory- based intervention that combines web, mobile phone and social media components into an engaging weight control program. The project will build on our prior work in intervention research to improve weight-related behaviors and weight outcomes and in mobile phone based behavioral interventions. We will perform formative research on SMART in San Diego and in collaboration with researchers at Stanford University. This will be followed by development of the final SMART software architecture and system. Then a 2-year randomized controlled trial will be conducted to evaluate the effects of SMART on weight-related outcomes. The primary aims of the study will be differences in weight status (kg) at 24 months among two groups: a) Those with a normal weight at baseline to evaluate how well SMART supports the prevention of weight gain;and b) Overweight/obese participants at baseline in whom weight loss will be evaluated. Sufficient numbers of participants in both groups will be recruited to evaluate these outcomes. Secondary and exploratory aims of the study will be to examine other health related outcomes and process measures related to intervention use and satisfaction. RELEVANCE (See instructions): Overweight/obesity are highly prevalent conditions in the US impacting approximately 66% of the adult population. Reducing the overall incidence of obesity is a major public health concern, yet little is known about how to do this with young adults. This study will address this gap in knowledge. (End of Abstract)